As a result of a rapid increase in an amount of traffic of a backbone communication system due to the prevailing Internet, it is desired to develop a practical ultra high speed optical communication system beyond 40 Gbps. As an art which realizes such an ultra high speed optical communication system, the optical phase modulation method and the optical polarization multiplexing/demultiplexing art are noted.
The optical phase modulation method different from the optical intensity modulation method carrying out data modulation of an optical intensity of a laser beam, carries out data modulation of a phase of the laser beam. As the optical phase modulation method, a method of QPSK (Quadruple Phase Shift Keying) or 8 PSK is well known. According to the optical phase modulation method, it is possible to lower a symbol rate (baud rate) by assigning a plurality of bits to one symbol. For this reason, since it is possible to lower a working speed of an electric device by use of the optical phase modulation method, it is expected to reduce a manufacturing cost of an apparatus.
For example, in the case of the QPSK method, 2 bits (for example, 00, 01, 11 and 10) are assigned to four optical phases (for example, 45 degrees, 135 degrees, 225 degrees and 315 degrees) respectively. As a result, a symbol rate of QPSK is reduced to a half of a symbol rate (that is, bit rate) of the light intensity modulation method.
FIG. 5 exemplifies a constellation and a symbol mapping of QPSK. The constellation is a figure showing a symbol and a bit stream assigned to the symbol, on a phase plane. Four symbols of QPSK are shown in FIG. 5. Moreover, FIG. 5 shows also the bit stream (00, 01, 11 and 10) associated with each the symbol in the optical phase modulation method. It is called the symbol mapping to associate the bit stream with each the symbol as mentioned above. Here, in FIGS. 5 to 7, a horizontal axis and a vertical axis are an I (inphase) axis and a Q (quadrature) axis respectively.
Hereinafter, an optical communication method which uses QPSK as the optical phase modulation method will be exemplified. However, even if another optical phase modulation method is used, the following description is applicable to the optical phase modulation method.
In order to receive an optical signal based on the optical phase modulation method, the optical coherent reception method is used. In the optical coherent reception method, a signal light and a laser beam which has almost the same optical frequency as the signal light has (called as a local oscillation light or a local light) are combined by an optical element called a 90 degrees hybrid. Then, a light receiving element which receives an output of the 90 degrees hybrid outputs a beat signal between the signal light and the local light. Since the basic art of the optical coherent reception method is well known, only an outline will be described below.
To make description simple, it is supposed that polarizations of the signal light and the local light are the same linear polarization type. In the case that the optical coherent reception method is used, an alternating current component of an electric signal, output by the light receiving element, is the beat signal between the optical signal and the local light. The amplitude of the beat signal is proportional to strength of the signal light and strength of the local light. If a frequency of an optical carrier wave of the optical signal and a frequency of the local light are identical with each other, a phase of the beat signal is a phase difference between a phase of the signal light and a phase of the local light. Meanwhile, if the phase of the local light is identical with an optical phase of an optical carrier wave (laser beam) input to an optical modulator of an optical transmitter, the phase of the beat signal is identical with the phase of the laser beam in the optical transmitter. For this reason, by carrying out the symbol mapping to the phase of the beat signal, and converting the phase of the beat signal into a bit stream, it is possible to regenerate transmission data with which the optical carrier wave is phase-modulated in the optical transmitter. That is, if the optical transmitter transmits an optical signal which has the constellation shown in FIG. 5, it is possible that the optical receiver regenerates signals which have the similar constellation.
However, in general, the frequency of the optical carrier wave of the signal light and the frequency of the local light are not identical perfectly. Furthermore, the phase of the local light in the optical receiver is not identical usually with the phase of the laser beam input to the optical modulator in the optical transmitter. An optical phase difference between the optical carrier wave input to the optical modulator in the optical transmitter and the local light in the optical receiver is called an optical phase deviation. Moreover, a difference between the frequency of the optical carrier wave of the optical signal and the frequency of the local light is called an optical carrier wave frequency deviation (hereinafter, referred to as “optical frequency deviation”).
FIG. 6 shows a constellation of a QPSK signal in the case that the optical phase deviation exists. In the case that the optical phase deviation exists, signals whose constellation is rotated by an angle corresponding to the optical phase deviation in comparison with the constellation shown in FIG. 5, are received. It is impossible to know a value of the optical phase deviation in advance. For this reason, when carrying out the symbol mapping shown in FIG. 5 just as it is to the data shown in FIG. 6, and converting into the bit stream, false data may be regenerated.
Furthermore, in the case that the optical frequency deviation exists, the phase of the above-mentioned beat signal has a value obtained through adding the optical phase deviation to a product of the optical frequency deviation and a reception elapse time. For this reason, in the case that the optical frequency deviation exists, as shown in FIG. 7, signals whose constellation shown in FIG. 5 is rotated temporally are received. Since the phase of the beat signal continues to change temporally in this case, it is impossible to regenerate data on the basis of the phase of the beat signal by use of the symbol mapping shown in FIG. 5.
Accordingly, in the case of the optical phase modulation method, a function to compensate the optical phase deviation and the optical frequency deviation is mandatory in order to reduce influence caused by rotation of the constellation due to the optical phase deviation and the optical frequency deviation. Hereinafter, a process of compensating the optical phase deviation and the optical frequency deviation used widely in the optical phase modulation method will be described.
Each of FIG. 8 and FIG. 9 shows a configuration of a compensation circuit compensating the optical phase deviation and the optical frequency deviation. Here, the compensation circuit compensating the optical phase deviation and the optical frequency deviation may be denoted as “compensation circuit” in some cases. A compensation circuit 201 shown in FIG. 8 is called the feed forward type and a compensation circuit 301 shown in FIG. 9 is called the feedback type. The feed forward type compensation circuit carries out the phase compensation of an output signal so that the phase deviation detected from an input signal may be cancelled. The feedback type compensating circuit carries out the phase compensation of an input signal so that a phase deviation detected from an output signal may become small. The operation to detect the phase deviation is basically common to both the compensation circuits. Accordingly, as an example of the compensation circuit, the feed forward type compensation circuit 201 shown in FIG. 8 will be described in the following.
According to FIG. 8, an input signal input to the compensation circuit 201 is branched in two directions, and one branched input signal is input to a phase compensation amount estimating unit 100, and the other branched signal is input to a compensation executing unit 101.
The phase compensation amount estimating unit 100 includes a phase error detecting part 102, a filter part 103 and a phase compensation amount calculating part 104. The phase error detecting part 102 detects a change of the optical phase deviation per a unit time, that is, a change in the optical phase deviation between two adjacent symbols. As algorithm for detecting the change in the optical phase deviation in the optical coherent reception method, for example, the M-th Power Algorithm is known. Since the configuration and the procedure for applying the M-th Power Algorithm to the optical coherent reception method are known widely, detailed description is omitted.
An output of the phase error detecting part 102 is sent to the filter part 103. The filter part 103 removes a noise component from the output of the phase error detecting part 102. An output of the filter part 103 is sent to the phase compensation amount calculating part 104. The phase compensation amount calculating part 104 calculates the actual amount of phase compensation, that is, the amount of rotation of the constellation. For example, the phase compensation amount calculating part 104 is an integration circuit.
The compensation executing unit 101 carries out the phase compensation, that is, the phase rotation whose amount is corresponding to the amount of phase compensation detected by the phase compensation amount estimating unit 100, by executing a complex multiplication so that the detected optical phase deviation may be small, and outputs a signal whose phase has been compensated, as an output signal. Specifically, the compensation executing unit 101 outputs a product of the input signal and a complex value (expressed by exp (−i φ)) which means reverse rotation by φ, where φ is the calculated phase deviation. In this way, the phase deviation of the input signal is compensated.
Here, the change in the optical phase deviation between two adjacent symbols is equal to a product of the optical frequency deviation and one symbol time (one symbol time is equal to a reciprocal of a symbol rate), and one symbol time is fixed. For this reason, by the compensation circuit 201's calculating the optical frequency deviation on the basis of the optical phase deviation, and making the phase of the input signal rotated so as to compensate the calculated optical frequency deviation, it is possible to make the compensation executing unit 101 compensate the optical frequency deviation. That is, by adding a procedure in which the phase compensation amount estimating unit 100 calculates the optical frequency deviation on the basis of the optical phase deviation, it is also possible that the compensation circuit 201 compensates the optical frequency deviation.
As described above, according to the optical communication system based on the optical phase modulation method, by compensating rotation of the constellation by use of the compensation circuit 201 or 301 shown in FIG. 8 or FIG. 9 respectively, it is possible to regenerate the transmission data on the basis of the phase-modulated signal data. Since the compensation circuits 201 and 301 shown in FIG. 8 and FIG. 9 respectively are generally realized by carrying out the digital signal processing, the optical phase modulation method is often called the optical digital coherent method.
Meanwhile, as an art which realizes the ultra high speed optical communication system, the optical polarization multiplexing/demultiplexing art is also noted. An optical transmitter is based on the optical polarization multiplexing/demultiplexing art, modulates two independent optical carrier waves which have the same frequency band and whose polarization states are orthogonal to each other, with two different optical signals, and carries out the polarization multiplexing scheme and transmits the polarization-multiplexed signal. Moreover, an optical receiver based on the optical polarization multiplexing/demultiplexing art regenerates two independent signal data by demultiplexing the carrier waves whose polarization states are orthogonal to each other and as the result, the optical receiver makes it possible to realize a double transmission rate. Or, since it is possible to lower the symbol rate (baud rate) up to half of the bit rate of the transmission data by use of the optical polarization multiplexing/demultiplexing art, it is possible to reduce the operation speed of the electric device. For this reason, it may be proper to say that the optical polarization multiplexing/demultiplexing art is an art which can reduce an apparatus cost, among the optical communication systems of the identical symbol rate.
By combining the optical phase modulation method and the optical polarization multiplexing/demultiplexing art described above, it is possible to realize the ultra high speed optical communication system such as, for example, 100 Gbps system. Furthermore, an art to demodulate a received optical signal with accuracy by carrying out a process of compensating the optical frequency deviation, and the optical phase deviation and a process of demultiplexing the polarizations by use of a digital signal processing circuit installed in LSI (Large Scale Integration) or the like, is proposed.
A configuration that an optical receiver of the optical communication system based on a combination of the optical phase modulation method and the optical polarization multiplexing/demultiplexing art, uses the above-mentioned compensation circuit will be described with reference to FIG. 10 and FIG. 11. FIG. 10 shows a configuration of the feed forward type compensation circuit used widely in the optical digital coherent reception method. Moreover, FIG. 11 shows a detailed configuration of optical frequency deviation compensating circuits 201-1 and 201-2 shown in FIG. 10.
A polarization multiplexing optical signal input into the optical receiver, is polarization-demultiplexed into a X polarization wave and a Y polarization wave orthogonal to each other, by a 90 degrees hybrid (not shown in the figure) of a front end unit of the optical receiver. Then, optical signals of each polarization are mixed with a local light to generate beat signals. Here, since two polarization planes of the input polarization multiplexing light are generally not identical with the polarization planes of the 90 degrees hybrid, each of the beat signals includes both of the multiplexed polarization signals. A polarization demultiplexing circuit 200 shown in FIG. 10 and FIG. 11 demultiplexes the beat signals (X polarization input signal (A) and Y polarization input signal (A)) into signal data corresponding to two optical carrier waves whose polarizations are orthogonal to each other. As signal processing algorithm used in the polarization demultiplexing circuit 200, for example, CMA (Constant Modulus Algorithm) is known. Since a procedure for applying CMA to the polarization demultiplexing process is well known and has no direct relation to the configuration of the present invention, description on the procedure is omitted.
The signal data polarization-demultiplexed by the polarization demultiplexing circuit 200 are input to the optical frequency deviation compensating circuits 201-1 and 201-2 respectively to compensate rotation of the constellation. As shown in FIG. 11, the optical frequency deviation compensating circuits 201-1 and 201-2 include compensation amount estimating units 100-1 and 100-2, and compensation executing units 101-1 and 101-2 respectively. Each of the compensation amount estimating units 100-1 and 100-2 calculates the amount of carrier frequency compensation by calculating the optical frequency deviation on the basis of the optical phase deviation similarly to the phase compensation amount estimating unit 100 described in FIG. 8. Then, the compensation executing units 101-1 and 101-2 compensate the phases of the signal data input to the optical frequency deviation compensating circuits 201-1 and 201-2 respectively on the basis of the calculated amounts of carrier frequency compensation.
That is, the optical frequency deviation compensating circuits 201-1 and 201-2 compensate rotation of the constellation caused by a frequency difference (that is, wavelength difference) between a light source of the optical transmitter and the local light of the optical receiver.
The signal data whose optical frequency deviations are compensated are input to optical phase deviation compensating circuits 202-1 and 202-2 respectively. The optical phase deviation compensating circuits 202-1 and 202-2 compensate fluctuation of the constellation which is not caused by the optical frequency deviation, and which is caused, for example, by fluctuation of the phase of the light source of the optical transmitter, or fluctuation of the phase of the local light of the optical receiver. Since operation of the optical phase deviation compensating circuits 202-1 and 202-2 for compensating the phase deviation are similar to the operation of the compensation circuit 201 shown in FIG. 8, description on the operation is omitted.
As mentioned above, the feed forward type compensation circuit shown in FIG. 10 and FIG. 11 which uses a combination of the optical phase modulation method and the optical polarization multiplexing/demultiplexing art, compensates the optical frequency deviation and the optical phase deviation of each of two independent optical signals obtained by the polarization demultiplexing. As a result, it is possible that the feed forward type compensation circuit shown in FIG. 10 and FIG. 11 realizes an ultra high speed optical communication system such as a 100 Gbps system.
Here, a patent document 1 and a patent document 2 related to the present invention, disclose an art of carrying out correction of a phase of an optical phase-modulation signal and demodulation of the phase-modulation signal.